Tighten to yield bolts - Re-use
 

I recently bought a 496HO which had been underwater. It sank whilst not running.

It sat for a while before I got it so had some corrosion.

I have pulled everything to bits and replaced lots of parts.

I am in Australia so parts availability has been scarce and most came for the US.

I machined the crank 10 thou and bored the block 30 thou. I put in new bearings, new pistons (forged) but reused the original rods and big end bolts and nuts mainly because I could not get replacements at the time and wanted the boat going for our summer.

I used new head bolts as they were easy to source.

In assembly I had to tension the big end bolts several times as I had to remove the crank twice.

My questions are:

Does anyone have experience in re-using these original mercruiser conrod bolts which are intended for single use? If so with what results?

Are the nuts and bolts available separately anywhere?

The motor is in stock form (new original lifters, cam etc) other than the 30 thou oversize forged pistions.

I would hate for it to fly to bits because of a cheap part. I am considering replacing the rods (if the original bolts are not available separately) and any suggestions on availability, part numbers, suppliers for suitable rods would be appreciated.

Might try www.jegs.com or summitracing.com not a marina but tons of race stuff i have bought plenty of stuff from both some for boat some for cars they might have what you are looking for

ARP makes all sorts of bolts for hi performance engines- I am sure they have something for you. Check the ARP catalog.

I have built a lot of motors with torque to yeild bolts, mostly automotive. I usually replace anything on the rotating assembly, but have bypassed replacing the head bolts (TQ to yeild.) For peace of mind, i would plan on replacing them at the end of the season, unless they were hardened aftermarket bolts/studs.

You can purchase new nuts for the rod bolts as GM recommends from any Chevrolet-GMC truck dealer parts of GM Parts Direct.com. as rod bot nuts for any 2500-3500 series truck with the 8.1Liter engine. ( same basic engine as GM Powertrain supplies to Mercury.
They are angle torque spec'd but you can torque them to 53 ft/lbs with lube safely with a good calibrated torque wrench.

Best Regards,
Ray @ Raylar

Can you get the bolts as well or only the nuts? I would have thought the bolts would break after stretching too much and didn't think the nuts would fail?

Also I looked on teh GMParts Direct site and could not find the bolts as a separate item, - only with the conrod assembly. Am I looking in the wrong spot?

Thanks for your help and can I just say Ray from Raylar, I have read heaps of your posts on the 496 and your product knowledge is unreal! Keep up the informative posts because I for one certainly appreciate it.

Definitely don't re-use them. You're right, they stretch.

Just to let Merc 496 owners know at Raylar we do not re-use the stock 8.1L connecting rods in our rework of higher performance 496 versions. Raylar replaces with new billet H-beam rods for strength and forged piston sets to match so we do not replace stock rod bolts or nuts.
I have been told that Volvo-Penta recommends from their conversations with GM Powertrain to just replace the rod nuts only and they supposedly stock an sell them as a seperate part.
As for rod bolts stretching , yes they do, all rod bolts stock and aftermarket stretch to place and hold the correct loads and pull on rod caps to keep rod ends and bearings where they need to be. That is why we an others use rod bolt stretch measuring tools to measure rod bolt stretch before and after assembly to make sure we get the stretch correct as torquing is just a close approximation of this required stretch in all rod bolts.
There is good informational and training reading at ARP bolt companies website on how to use, measure, stretch and torque rod bolts properly and the reasons for this procedure.
This should be a must read for anyone who is unfamiliar with proper rod bolt procedures and who is attempting to re-build or build high performance engines.
You could measure the exact length of your rod bolts as torqued in place by GM and then after they are removed and then see if at 53lbs they stretch to about the same length and return to the un-torqued length when the nuts are loosened. If they return to their un-torqued length or stretch length then they are not torque to yield -must replace bolts.

Best Regards,
Ray @ Raylar

GM Service documentation says to replace only the nuts.

I would like to try to explain a common misconception about TTY bolts.

1. Not every bolt that is tightened using the "torque + angle" method is a Torque To Yield bolt. There are pleny of fasteners that are tightened using a "torque + angle" that do not need to be replaced. Several manufacturers have used this method of tightening for quite a few years on head bolts yet the bolts do not need to be replaced because they are not tightened to the yield point. A perfect example in the High Performance world are the bolts used in Oliver rods.
Unless the manufacturer stipulates bolt replacement there is absolutely no reason to replace them.

Hope this helps,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

There ya go. Some do stretch. I got a demo from a mechanic years ago where we measured the length of torque to yield head bolts and the used ones were longer. You don't want to re-use those. Go by what the manufacturer says. I'd put in good bolts anyway though.

bill and raylar are correct in all respects but in the simplest terms it, just so you know, it works like this. all steel stretches when stressed and when you tension any piece of hardware, that's what you are doing... stressing it. the stretch , in engineering terms is called strain. every steel has a " stress vs strain curve" you load it...it stretches. you load it some more and it stretches some more. and if you release the load it goes right back to where it was. however if you stress it too far it goes past its " elastic limit" and into a " plastic deformation stage" commonly called " yield" . at this point on the curve, the steel accepts no more load but just continues to stretch and it will NOT return to its original length when the load is released. what has happened is that the actual molecular structure of the steel has been damaged and that's the end of that. attempting add more load ( stress) just results in a a lot of stretch followed by failure. on a connecting rod bolt, the spec you are given for " torque" is supposed to be the number that with properly lubricated bolts, will stretch the steel to a design preload on that stress vs strain curve to give you the clamp load you need to hold the pieces together.

if the hardware is true " torque to yield" and it has, in fact been there, then that hardware is a throw away after a single use. but like a lot of things, that term gets thrown around so much that people and even manufacturers use it as shorthand. so the practical answer is simple... do what the manufacturer of the bolts tells you to do. he knows what he meant when he used that term. if GM says its ok to reuse the bolts w/ new nuts, then don't try to interpret the semantics, just go ahead and do it. if they say throw away the bolts after a single use, then believe them. if you buy aftermarket bolts you follow those instructions to the letter for the same reason... especially the lube part. the parasitic friction of the assy can make your torque numbers lie to you comprehensively. thats why the stretch method is absolutely the best where it is possible to use.